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Full text of "The Cambridge Natural History"

106                    PROTOTHERIA — MONOTREMATA  "                 CHAP.
the latter are only known from very fragmentary remains,
which are not sufficient to determine the systematic position of
the animals of which they are fragments, I have not thought
it worth while to attempt a serious definition of the order
Multituberculata. I have introduced a short account of the
principal facts which are known concerning the creatures
grouped together under this name into the historical sketch of
the progress of mammalian life In Chapter IV. As to the
Monotremata, there Is no question that they are entitled to rank
In a group equivalent to that including all other mammals of
which we have sufficient knowledge to construct a classificatory
scheme. There have been. Indeed, naturalists, such as Meckel,
who would altogether deny the mammalian rank of these
creatures.
The Monotremata or Ornithodelphia may be thus defined:—
Mammalia with no teats, but with a temporary pouch In.
which the young are hatched, or to which they are transferred
after hatching, and into which open the ducts of the mammary
glands. An anterior abdominal vein, or at least the membrane
supporting it, persists throughout the abdominal cavity. Heart
with an incomplete and largely fleshy right auriculo-ventricular
valve. Brain without a corpus callosum. Shoulder girdle with
a large coracoid reaching the sternum ; clavicles and an inter-
clavicle present. There are " marsupial" or epipubic bones
attached to the pelvis. "Vertebrae with no epiphyses for the
most part. Ribs with only eapitulum. and no tuberculum.
Mammary glands of the sudoriparous and not the sebaceous
type of epidermic gland.1 Oviparous, with a large-yolked and
ineroblastic ovum, enclosed within a follicle of two rows of cells.
To call these animals Mammalia is of course an abuse of the
meaning of that word In one sense, but it is not in another;
since the pouch of these Monotremes is, as has been explained
elsewhere (p. 16), the real equivalent of a teat, and not of the
pouch of the Marsupials.
The most salient characteristic of this group of mammals in
the estimation of their position in the vertebrate series is not
so much the fact that they are oviparous as that the eggs are
large-yolked, and develop therefore, so far as regards their early
stages, after the fashion of the egg of a reptile. The laying
1 Gtegenbaur, Zur J£enn(ni$s d&r Mammarorgane der Movwlremevi, Leipzig, 1886.